Our ongoing studies of aging rats demonstrate that the levels of monoamine neurotransmitters are altered in discrete brain regions and correlate some of these changes to endocrine alterations. For example, the loss of cyclicity in brain serotonin is associated with failure of gonadotropin (LH) surge. Maturation and aging of the reproductive function have been accelerated by perinatal pharmacological blockade of serotonin. In other experiments, immature rats maintained on severely restricted typtophan diets (a dietary regimen previously shown to delay aging) had reduced levels of brain serotonin. Currently, several diets of increasing degree of tryptophan restriction permit chronic maintenance of animals with less pathology and mortality than with previous diets and may enable us to identify those conditions and factors most favorable for survival. Our efforts to modify aging by intervention on brain neurotransmitters, their enzymes or precursors have led us to feed aging rats with diets high in tyrosine to influence brain catecholamines. We have found that this diet restores estrous cyclicity in some middle-aged female animals. We plan to continue our analysis of the roles of the central nervous and endocrine systems in aging utilizing continued biochemical analysis of brain neurotransmitters, analysis of pituitary hormone polymorphism, isotopic techniques to visualize the distribution of neurotransmitter and polypeptide amino acid precursors such as tryptophan in control, nutritionally-deprived and pharmacologically treated animals.